Mucoid Pseudomonas aeruginosa strain FRD, a sputum isolate from a cystic fibrosis (CF) patient, was used to develop a genetic system. The mucoid appearance is due to the biosynthesis of the expolysaccharide and is a potential virulence factor of the organism in respiratory infections of CF patients. Sex-factor plasmids FP2 and R68.45 were used in genetic exchange experiments which demonstrated that alginate (alg) genes which undergo frequent mutations are chromosomal, that several genes are involved, and that these genes are apparently clustered on the chromosome. Analysis of these genes by transduction and molecular cloning with plasmid vectors is in progress. Studies on the physiology of mucoid P. aeruginosa showed that the sputum of CF patients supports both growth and alginate biosynthesis. P. aeruginosa in sputum-medium is inhibited by low concentrations of mannose, both in terms of growth and alginate yields, suggesting its efficacy in respiratory therapy. Physiological differences between mucoid strains and their non-mucoid derivatives are under investigation.